Meiosis generates haploid gametes, such as sperm and eggs, from a diploid cell such that a diploid genome is restored upon fertilization. The proper segregation of chromosomes during the meiotic divisions depends on events in meiotic prophase, such as synapsis of homologous chromosomes and crossover recombination. Errors in chromosome segregation are usually fatal to the fertilized zygote but can also result in cancer predisposition or developmental disorders. We have identified a meiotic checkpoint that responds to defects in homolog synapsis, independent of a DNA damage/recombination checkpoint, and activates apoptosis to avoid the generation of aneuploid gametes. Not all unsynapsed sequences have the capacity to trigger this checkpoint; rather, this pathway is specifically activated by unsynapsed Pairing Centers (PCs), chromosome sites that promote synapsis in C. elegans. Furthermore, the checkpoint requires the C. elegans homolog of PCH2, a budding yeast meiotic checkpoint gene, suggesting that the molecular mechanism that detects synaptic failure is conserved. Using a combination of genetic, biochemical and cytological approaches, we plan to further investigate the synapsis checkpoint. We will address how chromatin state(s) contributes to the ability of PCs to activate the synapsis checkpoint by studying the role of chromatin-modifying enzymes at these cis-acting sites. We will determine how the assembly of the synaptonemal complex (SC) is monitored during a normal meiosis by localizing PCH-2 in wildtype and mutant backgrounds, identifying proteins that interact with PCH-2 and investigating whether PCH-2 specifically modifies an important class of SC components. Furthermore, we will identify additional components of the checkpoint by undertaking an RNA interference screen that will focus on candidate genes that fulfill specific expression and phenotypic profile criteria. This screen has identified a putative transcription factor as a checkpoint component and we will test whether this factor directly regulates the core apoptotic machinery in response to checkpoint activation. These complementary approaches will enable us to gain a molecular and mechanistic understanding of how homolog synapsis is monitored during meiosis and how an unsynapsed or inappropriately synapsed homolog generates a checkpoint signal that is ultimately translated into an apoptotic response to avoid the production of aneuploid gametes.